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What is Circuit Switched FallBack (CSFB) ?

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What is Circuit Switched FallBack (CSFB) ?

Circuit Switched FallBack (CSFB) is a technology used in 4G LTE whereby voice and SMS services are delivered to LTE devices through the use of GSM or another circuit-switched network.

Circuit Switched FallBack is needed in some networks because LTE is a packet-based all-IP network that cannot support circuit-switched calls. When an LTE device is used to make or receive a voice call or SMS, the device “falls back” to the 3G or 2G network to complete the call or to deliver the SMS text message.

While making a voice call may seem simple, largely depends on the scenario where the user is, and alternatives available for its completion. So it is necessary to understand well what are the possibilities and the most important concepts of these key scenarios.

In the first generation of cellular networks, the communication through voice calls was the main goal, and was based on a circuit switched topology or ‘channels’ (CS Circuited Switched).

Over time, the need for other services (data!) has emerged. Voice calls have come into existence with these new services. As demand increased, these new services were supported by a new domain, the IP-based packet-switched (PS Packet Switched).

CSFB was specified in 3rd Generation Partnership Project (3GPP) Release 8. CSFB requires a software upgrade of the operators core and radio network.

CSFB is often seen as an interim solution for LTE operators:  Voice-over-LTE (VoLTE) is considered to be the long-term goal for the delivery of voice services on LTE networks.

LTE CSFB Illustration
LTE CSFB Illustration

Voice services in LTE

In order for we to have voice services in LTE, changes need to be made. There are some alternatives, and below we have the main ones:

  • VoLGA (Voice over LTE via Generic Access): Use legacy 2G/3G as a generic access, ‘packaging’ voice services, and delivering via LTE.
  • CSFB (CS Fall Back): whenever the UE have the need to place a call, make it revert (fallback) for legacy networks.
  • VoLTE (Voice over LTE): make voice over LTE itself. In this case, the voice is pure IP – VoIP LTE.
  • SRVCC (Single Radio Voice Call Continuity): ensure that purely LTE (VoLTE) calls are transferred (via handover) to the legacy networks in a transparent manner.

To deliver voice services in LTE network is necessary to have an infrastructure that support it. In other words, there needs to exist an IMS (IP Multimedia Subsystem or IP Multimedia Core Network Subsystem). If an IMS is available, then the voice over LTE may be provided as long as a minimum set of IMS functionality and entities also are present.   As a note: IMS is much more complete, and have more other purposes than the voice. The voice is just another ‘application’ of IMS.

IMS

IMS is a backbone (network) at the application level, which works on top of other wireless networks and not just the LTE (as 3G, 2G, WiFi and others).  The concept is quite broad, and to understand it with all its entities, possibilities, interfaces, protocols, and possibilities is an extremely difficult task, even for the most experienced in the subject.  The IMS is not new: it already existed before the LTE (as well as other entities, such as the EPC PRCF, which also is not new!). Its complete specification consists of thousands and thousands of 3GPP standards. But let’s try to understand in a simpler way than that found there.

As its name suggests (IP Multimedia Services), IMS offers several multimedia IP services, including VoIP (Voice over IP). In IMS, voice is just ‘another’ service! IMS brings together voice features such as authentication, service authorization, call control, routing, interoperability with PSTN, billing, additional services and VAS. None of these exist in the EPC: this is the reason why the pure EPC without IMS can not process a voice call.   For VoLTE, access is made by the SAE (eUTRAN + EPC), while voice service lies in the IMS.

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